1. Field of the Invention
The present invention relates to interposers for connecting land grid array components. More specifically the present invention relates to an interposer with conductors disposed at a small pitch.
2. Discussion of Background Information
Circuit boards ordinarily include circuit patterns or traces printed on various levels internal to the board and/or on one or both sides of the board. Components are ordinarily mounted on one side of the board and electrically connected to the circuit patterns. Prior to the introduction of surface mount and land grid array components, it was common to mount components to a circuit board and extend leads from the components through plated holes in the circuit board to mounting pads on the opposite side. The plated-through holes were electrically connected to the desired locations on the circuit pattern. The component leads were ordinarily arranged in a pattern along the edges of the component with the ends of the component leads soldered to the pads on the opposite side of the board to establish mechanical and electrical connection to the board and circuit pattern.
Surface mounted components were developed for mounting and connecting to one side of the board. In one form of surface mounted components, leads extend from along the edges of the component for attachment to pads on the circuit board. In another form of surface mounted components, compliant leads were attached to pads along the edges of the component and to pads on the circuit board.
Land grid array components are a class of surface mounted components that employ a grid array of solder connections on a surface of a component. The solder connections of the grid array are not limited to the edges of the component, but instead are dispersed across the surface in spaced relation in an area array that is usually uniform. Land grid array components include ball grid array components, solder grid array components and column grid array components. The surface bearing the land grid array is positioned to confront an identical grid array of circuit pads on a circuit board, and solder beads, balls or columns are heated to reflow to attach or detach the component from the circuit board.
One difficulty with land grid array attachment techniques is that inspection of the solder connections cannot always be accomplished. More particularly, while solder beads and connections at the edges of the array might be visually inspected, it has been difficult to inspect connections at the interior of the array.
To overcome the above drawbacks, U.S. Pat. No. 5,953,816 provides a flexible interposer that would allow for laser drilling of holes for conductive materials, where the electrical connections were at a 0.040-0.050 inch pitch. In the intervening ten years since the '816 patent was filed, technical advancements have reduced the pitch between holes to about 0.012-0.022 inches. It is difficult to apply the techniques of the '816 patent to this smaller pitch size because the polyimide mask concentrates stress on the solder joints between the interposer and the attached electrical components.